The technology of vector-controlling a permanent magnet synchronous motor by use of an inverter is widely utilized in the industrial fields; by separately operating the amplitude and the phase of the output voltage of the inverter, the current vector in the motor is optimally operated so that the torque of the motor is instantaneously controlled at high speed. Because, compared with an induction motor, magnetic flux is ensured by means of a permanent magnet, no excitation current is required, and because no current flows in the rotor, no secondary copper loss is produced; therefore, a permanent magnet synchronous motor is known as a high-efficiency motor, and the application of a permanent magnet synchronous motor to an electric vehicle control device has been studied in recent years. It is known that, in a magnet-embedded permanent magnet synchronous motor (i.e., interior permanent magnet synchronous machine, and abbreviated as IPMSM, hereinafter), which has been attracting people's attention in recent years, among permanent magnet synchronous motors, torque thereof is efficiently obtained by utilizing reluctance torque, produced through a difference between rotor magnetic resistance values, in addition to torque produced by magnetic flux caused by a permanent magnet.
However, it is known that, in an IPMSM, there exist a great number of combinations of d-axis current and q-axis current for generating given torque. Furthermore, it is known that the characteristics of an IPMSM such as the amplitude of a current that flows in the IPMSM, the power factor, the iron loss, and the copper loss largely change depending on the respective amplitudes of the d-axis current and the q-axis current, i.e., selection of the current vector. Accordingly, in order to operate an IPMSM efficiently, it is required to select an appropriate current vector in accordance with the application and operate it. That is to say, in a permanent magnet synchronous motor vector control device, it is required to generate an appropriate current command for instantaneously controlling the vector of an electric current that flows in a motor so that the current vector satisfies desired conditions described below; therefore, it is important in terms of configuring a system how to configure a current command generation unit that generates a current command from a torque command.
Methods of selecting a current command include a method of making the efficiency of a motor maximum, a method of making the power factor of the motor to be “1”, a method of making torque obtained with given interlink magnetic flux to be maximum, a method of making torque obtained with a certain electric motor current to be maximum, and the like; however, in terms of application to an electric vehicle control device, the method of making torque obtained with a given current to be maximum (referred to as “maximum torque control”, hereinafter) is optimal because, by utilizing this method, the current rating of an inverter can be minimized while the high-efficiency operation of a motor can be performed, whereby the loss in the inverter can also be minimized.
As a related conventional technology, Patent Document 1 discloses a method in which the respective optimal values of a d-axis current id and a q-axis current iq corresponding to various kinds of torque values of a motor are preliminarily measured and mapped; during operation of the motor, the map is referred to, as may be necessary, in response to a torque command, and a d-axis current command id* and a q-axis current command iq* corresponding to the torque command are obtained; then, current control is performed in such a way that the electric currents correspond to the d-axis current command id* and the q-axis current command iq*.
[Patent Document 1] Japanese Patent Application Laid-Open Pub. No. 2006-121855